Ion-exchangeable phosphate glass compositions and strengthened optical quality glass articles

ABSTRACT

Ion-exchangeable phosphate glass compositions containing in mole percent from about 50 to 70% P 2  O 5 , from about 5 to 30% Li 2  O, from about 5 to 25% MO, where M is selected from the group consisting of Be, Mg, Ca, Sr, Ba, and Zn, and about 5 to 30% X 2  O 3 , where X is selected from the group consisting of Al, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu are provided. In another aspect, the phosphate glass compositions of the present invention also contain in mole percent up to 10% R 2  O, where R is selected from the group consisting of Na, K, Rb and Cs. Solarization inhibitors and minor amounts of anhydrous fluorides and chlorides are also included in some embodiments. Optical quality phosphate glass articles formed of the phosphate glass compositions of the present invention are readily ion- exchangeable when contacted with certain salts. Optical quality phosphate glass articles are also provided having good thermal shock resistance. These glass articles have an inner tension region and an outer compressive surface layer formed using an ion exchange process. In some embodiments, laser rods and similar active optical elements are formed from the strengthened phosphate glass articles of the present invention where the optical elements are doped with an amount of a suitable dopant effective for laser activity.

This is a continuation of copending application Ser. No. 07/373,722filed on Jun. 29, 1989, now U.S. Pat. No. 5,053,360 issued Oct. 1, 1991,which is a divisional application of Ser. No. 07/128,676 filed on Dec.4, 1987, now U.S. Pat. No. 4,875,920 issued Oct. 24, 1989.

FIELD OF THE INVENTION

The present invention relates generally to phosphate glasses and morespecifically deals with phosphate glass compositions which have highthermal shock resistance. The present invention also relates to opticalquality phosphate glass articles such as laser rods and the like and tolaser devices which incorporate these glass articles.

BACKGROUND OF THE INVENTION

It is well-known that laser glasses must be able to withstand highinternal temperatures created by flash lamps and the like withoutexperiencing significant distortion, cracking or any significant changein optical properties. Commonly referred to as thermal shock resistance,optical elements such as laser rods, slabs, discs and fibers must becapable of enduring high-frequency optical pumping without catastrophicfailure. In operation, a laser glass element acquires heat from thepumping light source. In order to dissipate this heat, laser devices aretypically liquid-cooled which helps prevent thermal rupture of the laserglass element. However, laser glass rods and the like still developlarge internal temperature gradients, causing thermally-induced stressacross the element. At high powers and high repetition rates,conventional laser glasses may fracture as the result of large thermalgradients produced by internal optical pumping.

It is also known that thermal distortion of laser glass elementsproduces a variance in refractive index which may cause opticaldistortion of the laser beam. Hence, it is important to provide laserglasses which have a small change in refractive index over broadtemperature ranges. In some instances it is even desirable to providelaser glass elements having a negative change in refractive index topartially compensate for optical distortions produced by a positivecoefficient of thermal expansion. These thermal effects are also knownto diminish the energy efficiency of active laser components. Therefore,it would be desirable to provide glass compositions which could beformed into optical components having good mechanical strength and highthermal shock resistance for use as laser elements, filters and thelike.

A number of methods for strengthening silicate-based glasses have beenproposed, including ion-exchange or "stuffing" of the surface of asilicate glass article with large ions in exchange for small ions belowthe softening temperature of the glass. For example, one such method isdisclosed in "Stresses in Glass Produced By Non-Uniform Exchange ofMonovalent Ions," J. Am. Ceram. Soc. 45 [2] 59-68 (1962) wherein achemical method of silicate glass strengthening is disclosed whichinvolves low-temperature ion exchange. Other investigators, notablyNordberg et al., "Strengthening By Ion Exchange," J. Am. Ceram. Soc. 47[5] 215-219 (1964), have described low-temperature ion exchangestrengthening of silicate glasses. Ion exchange treatment of lithiasilicate laser glasses is also discussed in U.S. Pat. No. 3,687,799,entitled "Glass Lasers of Increased Heat Dissipation Capability Made ByIon Exchange Treatment of Laser Glass." However, very little researchhas been conducted on the strengthening of non-silicate glasses.

In U.S. Pat. No. 4,075,120 and U.S. Pat. No. 4,248,732, both of whichare assigned to the assignee of the present invention and which areincorporated herein by reference, novel phosphate glass compositionsparticularly suitable for forming laser glass elements are disclosed. Aswill be shown, the present invention provides phosphate glasscompositions which are unique in their ability to be ion-exchanged. Thatis, the phosphate glass formulations of the present invention produceunexpected superior results over the prior art.

In U.S. patent application "Ion-Exchangeable Germanate GlassCompositions and Strengthened Germanate Glass Articles," filed Oct. 27,1987, and assigned to the assignee of the present invention, novelgermanate glass compositions and articles formed of these germanateglasses are disclosed which exhibit good mechanical strength. However,although phosphate glasses have unique properties which make themsuperior to silicate, borate and most other glasses for certainapplications, to Applicant's knowledge, no one has successfullyion-exchanged a phosphate glass composition to produce optical qualitythermal shock resistant glass articles as provided by the presentinvention. Therefore, there exists a long-felt need for phosphate glasscompositions from which thermal shock resistant articles can be made andfor optical quality glass articles which exhibit good mechanicalstrength and which have a high degree of thermal shock resistance. Thepresent invention provides ion-exchangeable phosphate glass compositionsand optical quality strengthened phosphate glass articles which satisfythis need.

SUMMARY OF THE INVENTION

In accordance with the present invention, in one aspect there isprovided phosphate glass compositions which develop unexpected favorablethermal and optical properties when ion exchanged in accordance with thepresent invention. More specifically, the phosphate glass compositionsas provided herein produce unexpected superior results whenion-exchanged to form thermal shock-resistant glass articles.Preferably, the phosphate glass compositions of the present inventioncontain a mixture of P₂ P₅, Li₂ O, MO (where M is selected from thegroup consisting of Be, Mg, Ca, Sr, Ba and Zn) and X₂ O₃ (where X isselected from the group consisting of Al, Y, La, Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). In this embodiment, the preferredcompositional ranges in mole percentage are approximately 50 to 70% P₂O₅, approximately 5 to 30% Li₂ O, approximately 5 to 25% MO, andapproximately 5 to 25% X₂ O₃. It is to be understood that mixtures ofthe various species designated generally by X₂ O.sub. 3 are to beincluded in a single composition, as well as mixtures of the speciesdesignated generally MO. That is, an ion-exchangeable phosphate glasscomposition made in the accordance with the present invention may forexample include both BeO and MgO or BeO, MgO and CaO, with the combinedor total mole percentage being preferably maintained within thepreferred range for MO.

In another embodiment, the ion-exchangeable phosphate compositions ofthe present invention also include, in addition to Li₂ O which isessential to the invention, R₂ O, where R is selected from the groupconsisting of Na, K, Rb and Cs. The concentration in mole percent of R₂O in the phosphate glass composition preferably ranges from about 0.0 toabout 10%. Again, mixtures of the species designated by R₂ O such as Na₂O and K₂ O are suitable.

In another aspect, the present invention provides phosphate glassarticles exhibiting good thermal shock resistance and high mechanicalstrength which are particularly suitable for use in forming opticalelements such as laser glass rods and filters. These thermalshock-resistant phosphate glass articles have a relatively uniformchemical make-up except for the outer layer which, relative to the innerglass region, has a low concentration of lithium ions and a highconcentration of a preselected ion. That is, the articles have acompositional gradient such that the surface layer has a lowerconcentration of lithium than the inner regions and has a higherconcentration of a selected ion, other than lithium, than the innerregion. This will become more apparent during the explanation of thepreparation of the thermal shock-resistant phosphate glass articles ofthe present invention. The glass articles may be formed in a variety ofshapes and thicknesses and are particularly suitable for use as opticalelements such as laser rods and filters.

Hence, in still another embodiment, the present invention providesoptical elements which are formed from the thermal shock-resistantphosphate glass articles of the present invention. Thechemically-strengthened phosphate glass articles may be doped with anamount of a suitable dopant effective for laser activity for thefabrication of laser rods or the like. Optical discs or plates may alsobe formed. By providing thermal shock-resistant laser glass elements,glass fracture or rupture caused by large thermal gradients producedduring internal optical pumping is substantially reduced. Thus, byproviding thermal shock-resistant laser glasses, the present inventionallows laser rods and the like to be fabricated which can be used atmuch higher powers than conventional phosphate glass laser elements. Thethermal shock-resistant phosphate glass articles of the presentinvention are also favorably characterized by a small and sometimes anegative change in refractive index as the temperature of the glassarticle increases. Thus, in addition to reducing glass fracture, theglass articles of the present invention reduce distortion of the laserbeam when the laser glass elements are subjected to high pumping powers.

In still another aspect, the present invention provides a method forforming the thermal shock-resistant glass articles of the presentinvention. The method begins in a preferred embodiment with thepreparation of a phosphate glass article from the preferred phosphateglass compositions described herein. The phosphate glass article is thencontacted with a salt, preferably a molten alkali metal salt bath, at atemperature below the annealing temperature of the phosphate glassarticle to ion-exchange the surface layer. In this manner, an outercompression layer is formed by exchanging smaller ions present in thesurface layer with larger ions from the salt bath. This "stuffing" ofthe surface layer with large ions creates a compressive stress or outercompression layer in the phosphate glass article. Ion exchange isconducted under conditions which do not allow the compression layer torelax during cooling of the strengthened phosphate glass article. Hence,the cooled glass article has a surface layer in compression and a corein tension, producing high mechanical strength and good thermal shockresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the lasing slope efficiencies ofillustrative laser glass rods made in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment, the phosphate glass compositions of the presentinvention contain P₂ O₅, Li₂ O, one or more of the following compounds,BeO, MgO, CaO, SrO, BaO, ZnO, and one or more of the compounds, Al₂ O₃,Y₂ O₃, La₂ O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃, Pm₂ O₃, Sm₂ O₃, Eu₂ O₃, Gd₂ O₃,Tb₂ O₃, Dy₂ O₃, Ho₂ O₃ Er₂ O₃, Tm₂ O₃, Yb₂ O₃ and Lu₂ O₃. Forsimplicity, the preferred glass compositions may be expressed ascontaining P₂ O₅, Li₂ O, MO, where M is selected from the groupconsisting of Be, Mg, Ca, Sr, Ba, and Zn, and Combinations thereof, andfurther containing X₂ O₃, where X is selected from the group consistingof Al, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu andcombinations thereof. In the most preferred compositions the MOconstituent is predominantly BaO.

In another aspect, in addition to the above-stated constituents, thephosphate glass compositions of the present invention include one ormore of the compounds, Na₂ O, K₂ O, Rb₂ O and Cs₂ O. Again, forsimplicity, in this embodiment, the present invention may be expressedas containing P₂ O₅, Li₂ O, MO, where M is one is of the aforementionedM constituents, X₂ O₃, where X is one of the aforementioned Xconstituents and R₂ O, where R is selected from the group consisting ofNa, K, Rb, Cs and combinations thereof.

The addition of X₂ O₃ significantly increases the chemical durability ofarticles formed of the preferred phosphate glass compositions of thepresent invention, a factor which is important in decreasing the attackor corrosion of the glass articles during subsequent chemicalstrengthening in a molten salt bath. Also, in some applications, it ispreferred that R₂ O comprise predominantly K₂ O or another heavy alkalioxide. The heavier alkali oxides help reduce change in refractive indexwith temperature (dn/dT) of phosphate glass articles formed of theinventive compositions. It should be noted, however, that corrosion ofthe glass articles increases as the concentration of R₂ O increases andthus its concentration should be limited accordingly. The purity of thecomposition which is required will be dictated by the application of thefinished article, as will be appreciated by those skilled in the art.

Preferred phosphate glass compositions of the present invention whichdemonstrate unexpected superior ion exchange properties containapproximately 50 to 70 mole percent P₂ O₅, approximately 5 to 30 molepercent Li₂ O, approximately 5 to 25 mole percent MO, approximately 5 to25 mole percent X₂ O₃, and, where R₂ O is included, approximately 0.0 to10 mole percent R₂ O. Preferred are those phosphate glass compositionsof the present invention which have from approximately 55 to 65 molepercent P₂ O₅, approximately 10 to 20 mole percent Li₂ O, approximately10 to 15 mole percent MO, approximately 10 to 18, and more preferably,16 to 18 mole percent X₂ O₃, and, in those embodiments which alsoinclude R₂ O, approximately 0.0 to 5 mole percent R₂ O. The broad andpreferred concentration ranges are set forth respectively in Tables Iand II as follows:

                  TABLE I                                                         ______________________________________                                                  Approximate mole percent                                                      based on total glass composition                                    Compound  as oxide content before ion exchange                                ______________________________________                                        P.sub.2 O.sub.5                                                                         50-70                                                               Li.sub.2 O                                                                              5-30                                                                R.sub.2 O 0-10                                                                MO        5-25                                                                X.sub.2 O.sub.3                                                                         5-25                                                                ______________________________________                                    

                  TABLE II                                                        ______________________________________                                                  Approximate mole percent                                                      based on total glass composition                                    Compound  as oxide content before ion exchange                                ______________________________________                                        P.sub.2 O.sub.5                                                                         55-65                                                               Li.sub.2 O                                                                              10-20                                                               R.sub.2 O 0-5                                                                 MO        10-15                                                               X.sub.2 O.sub.3                                                                         10-18                                                               ______________________________________                                    

In another aspect, the phosphate glass compositions of the presentinvention include one or more solarization inhibitors. It will beappreciated by those skilled in the art that solarization darkens theglass, reducing its efficiency. Preferred inhibitors include Nb₂ O₅, Sb₂O₃, TiO₂, CeO₂, and MoO₃. Other suitable solarization inhibitors will beknown to those skilled in the art. Also, the phosphate glasscompositions of the present invention may include from about 0.01 molepercent to about 5.0 mole percent of solid anhydrous chloride and/orsolid anhydrous fluoride to aid in water removal. Preferred fluoridesare selected from the group of consisting of aluminum fluoride, lithiumfluoride, sodium fluoride, potassium fluoride and combinations thereof.Preferred chlorides are selected from the group consisting of aluminumchloride, lithium chloride, sodium chloride, potassium chloride andcombinations thereof. Melting the phosphate glass compositions under adry atmosphere or bubbling a dry gas through the glass melt will alsoaid in water removal from the glass. Conventional glass preparationtechniques can be used to form molten glass having the preferredcompositions, which is then cast into the desired shapes. The laserglasses of this invention may be prepared in accordance with recognizedpresent-day melt procedures. Preferably, the melting is accomplished inrefractory crucibles, such as fused quartz, zirconia and alumina, or inprecious metal crucibles, such as platinum and platinum alloys. Meltingof the glasses of this invention is accomplished at temperatures of fromabout 1500° F. to about 2300° F. Standard stirring and castingtechniques are employed to obtain the desired forms, including rods,disks, plates, fibers and other configurations associated with lasersolid state technology. Less common glass-forming techniques which willbe understood by those skilled in the art, such as the "sol-gel" processfor forming glass would also be suitable. The shaped phosphate glassarticle is then preferably ion-exchanged as will be described more fullyhereinafter.

As stated, the phosphate glass articles formed from the phosphate glasscompositions of the present invention are unique in their ability to beion-exchanged, which increases their mechanical strength and increasestheir thermal shock resistance. The inventive ion-exchanged phosphateglass articles of the present invention have an outer surface layer orregion which is modified with respect to the inner core such that theouter surface layer has a lower concentration of lithium ions than theinner region. Also, the outer surface layer has a higher concentrationof a selected ion or ions than the concentration of the selected ion inthe inner region. That is, the compressive stress surface layer hasessentially the same composition as the remainder of the glass exceptthat the concentration of lithium ions is less than the remainder of theglass and the concentration of ions of a different selected ion isgreater in the surface layer than in the remainder of the glass. Thepresence of these larger ions provides a compressive stress in thesurface layer relative to the inner tension region. This provides asubstantially strengthened phosphate glass article having high thermalshock resistance.

In a preferred embodiment, the outer surface layer contains a highconcentration of sodium ions with respect to the core or inner region ofthe glass article. In another embodiment the outer surface layercontains a higher concentration of potassium or potassium and sodiumions than the inner region. Most preferred are sodium ions whichexchange well with lithium ions present in the ion-exchangeable glasscompositions of the present invention. Other alkali metal ions may besuitable, such as rubidium or cesium, alone or in combination with theother preferred alkali metal ions, the concentrations of which are againhighest in the surface layer. Of course, lithium ions are not suitableas the selected ion since lithium ions are the smallest replaceable ionsin the preferred glass compositions.

It will be appreciated by those skilled in the art that theconcentration of ions which are substituted into the surface layer ofthe phosphate glass article of the present invention as well as thedepth of the ion-exchanged surface layer is determined by diffusion lawsand is thus in part a function of the length of exposure of the glassarticle to the source of ions to be diffused into the surface. Diffusionis also a function of the temperature at which the ion-exchange processis carried out. The monovalent ions which are removed from the surfacelayer of the phosphate glass article during ion exchange arepredominantly lithium ions. In the method of the present invention, thethermal shock-resistant phosphate articles of the present invention areformed by starting with a batch of glass having one of the preferredcompositions which is then cast in the conventional manner to formphosphate glass articles.

The phosphate glass articles so produced are ion-exchanged by contactingthe surfaces of the glass articles preferably with a molten salt bath.It may be possible to diffuse the preferred ionic species into the glassarticle surface using a suitable solvent, such as an organic solvent orthe like or by using other diffusion or ion implant techniques. The saltbath is preferably a molten alkali metal salt bath other than a lithiumsalt bath and, as stated, preferred alkali metal salts for use in thepresent invention are salts of sodium, potassium, rubidium, and cesium.The addition of some lithium to the alkali metal salt bath, however, maybe appropriate to control the rate of ion exchange or to lower themelting point of the alkali metal salt. The most preferred salt bathsare sodium salts, preferably sodium nitrate (NaNO₃).

In a preferred embodiment, the formed phosphate glass articles areion-exchanged by immersing them in a bath of the molten alkali metalsalt. Alkali ions in the salt bath diffuse into the surfaces of thephosphate glass articles where they are "exchanged" for smaller ionspresent in the glass surface. That is, the ionic radii of alkali metalions may be ranked in the following manner:lithium>sodium>potassium>rubidium>cesium. Thus, where the molten alkalisalt bath contains sodium ions, sodium ions replace lithium ions in theglass structure. This substitution at the surface of larger ions forsmaller ions sets up a significant stress or compressive layer at thesurface of the glass article. This high compressive stress outer layeroverlies an inner tension region or core of the glass article since nosubstantial exchange of ions occurs in the inner glass region. Thisproduces a significantly strengthened phosphate glass article havinghigh thermal shock resistance as demonstrated in the test data set forthherein. In those compositions of the present invention where bothlithium and sodium oxides are included, and wherein the salt bathcontains sodium ions, the sodium ions will replace both lithium andsodium ions in the glass. While the sodium-for-sodium exchange will notincrease compression, the sodium- for-lithium exchange will strengthenthe glass. It is theorized that depending upon the relative amounts ofsodium, potassium and lithium in a particular glass article, the glassarticle could still be strengthened in a sodium bath because of apreferential exchange of sodium for lithium. In lieu of sodium andpotassium, it may be suitable to diffuse other ionic species into thesurface layer such as silver ions. A preferred bath contains from about0 to 50% by weight NaNO₃ and from about 50 to 100% by weight KNO₃.Drying agents such as 1 to 10% aluminum powder may be employed to helpextend the usable lifetime of the bath.

One significant advantage of the present invention is that it providesphosphate glass articles which can be ion-exchanged at temperaturessufficiently below their annealing temperature so that distortion of theglass article which might otherwise occur during ion exchange isminimized. Thus, in one embodiment, phosphate glass articles formed fromthe preferred phosphate glass compositions provided by the presentinvention are polished or finished to form optical elements and are thenion-exchanged below their annealing point in the described manner. Inthis fashion, strengthened optical elements such as laser rods, slabs,and discs can be formed which are used to transmit or reflect light andwhich have high thermal shock resistance. That is, these compositionsallow chemical strengthening of active optical elements while minimizingdistortion, an important factor in optical glasses. Doping with aneffective amount of a dopant to provide good lasing properties iscarried out using conventional doping techniques. One preferred dopantis provided by Nd₂ O₃ to form neodymium doped glass lasers in accordancewith the present invention. Other suitable dopants will be known tothose skilled in the art. Methods of doping the inventive glass articleswith suitable dopant ions will be known to those skilled in the art.From about 0.03 to about 3.0 mole percent Nd₂ O₃ is preferred.

As stated, the temperature of the molten salt bath in which ion exchangeis conducted should be controlled such that the temperature of thephosphate glass article is maintained below its annealing temperatureduring ion exchange. That is, the surface layer of the phosphate glassarticle which is transformed into a compressive layer should always bebelow the annealing temperature of the glass article to preventrelaxation of the outer compressive stress layer. Also, the salt bathshould preferably be maintained at a temperature which is high enough tofacilitate fairly rapid diffusion of the selected ions into the glassarticle. The preferred temperature of the molten salt bath isapproximately 280 to approximately 410 degrees C., more preferablyapproximately 300 to approximately 380 degrees C., and most preferablyabout 320 degrees C. Temperatures outside these ranges may be suitablein a particular application. Also, where the phosphate glass article isground and polished, such as where an optical element will be formed, itis preferable that the polishing or finishing take place before theglass article is contacted with molten alkali metal salt bath.Conventional finishing and polishing techniques will be known to thoseskilled in the art.

Again, the depth and thickness of the compressive layer are determinedin part by the length of time which the phosphate glass article isexposed to the salt bath. Where the molten alkali metal salt bath ismaintained at a temperature which is within the preferred range asdescribed herein, it is preferred that the phosphate glass article becontacted with the salt bath for a period of approximately 1 toapproximately 500 hours, more preferably approximately 8 to 244 hours,and most preferably from approximately 16 hours to approximately 144hours. With these conditions, a compressive layer of approximately 10 toapproximately 400 microns and preferably from about 120 microns to about300 microns thick may be formed. A compressive surface layer of thesedimensions provides excellent mechanical strength and good thermal shockresistance. It may also be desirable to subject the phosphate glassarticle to an acid wash such as sulfuric acid prior to the step ofcontacting the surface with the salt bath to remove surfaceirregularities which are known to affect strength. Also, an acid washfollowing formation of the compressive layer may be desirable to removea very thin tension layer which sometimes forms overlying thecompressive layer.

While particular embodiments of this invention have been shown anddescribed herein, it will be understood, of course, that the inventionis not to be limited thereto, since many modifications may be made,particularly by those skilled in the art in light of this disclosure. Itis contemplated therefore by the appended claims to cover any suchmodifications as fall within the true spirit and scope of thisinvention.

The following examples are set forth to further illustrate and describethe present invention are not intended in any way to limit its scope.

EXAMPLES

Multiple phosphate glass compositions prepared in accordance with thepresent invention and having the preferred compositions as describedherein were prepared by batch melting using conventional heating,refining, stirring and casting techniques to form phosphate glassarticles. The samples were given reference numerals 1 through 21. Theconstituents in mole percent are set forth in the following TablesI-III.

                  TABLE I                                                         ______________________________________                                        Mole %                                                                        #1       #2      #3      #4    #5    #6    #7                                 P803     P781    P786    P787  P788  P789  P790                               ______________________________________                                        P.sub.2 O.sub.5                                                                     60     60      60    60    60    60    60                               Li.sub.2 O                                                                          17     17      15    16    17    17    17                               K.sub.2 O                                                                           --     --      2     1     --    --    --                               BaO   11.5   11.5    11.5  11.5  11.5  11.5  11.5                             Al.sub.2 O.sub.3                                                                    6.5    11.5    11.5  11.5  6.5   6.5   4.0                              Y.sub.2 O.sub.3                                                                     5.0    --      --    --    --    5.0   5.0                              La.sub.2 O.sub.3                                                                    --     --      --    --    5.0   --    2.5                              Nd.sub.2 O.sub.3                                                                    2.39   (2.31)* (2.33)*                                                                             (2.32)*                                                                             (2.52)*                                                                             (2.42)*                                                                             (2.53)*                          Nb.sub.2 O.sub.5                                                                    --     (0.48)* (0.49)*                                                                             (0.48)*                                                                             (0.54)*                                                                             (0.52)*                                                                             (0.54)*                          Sb.sub.2 O.sub.3                                                                    --     (0.20)* (0.23)*                                                                             (0.23)*                                                                             (0.24)*                                                                             (0.24)*                                                                             (0.25)*                          ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Mole %                                                                        #8        #9      #10     #11   #12   #13   #14                               P791      P793    P794    P795  P796  P809  P817                              ______________________________________                                        P.sub.2 O.sub.5                                                                     60      60      60    62    64    60    60                              Li.sub.2 O                                                                          15      19      19    17    15    7.5   16                              Na.sub.2 O                                                                          --      --      --    --    --    7.5   --                              K.sub.2 O                                                                           --      --      --    --    --    --    1                               BaO   13.5    11.5    11.5  11.5  11.5  13.5  11.5                            Al.sub.2 O.sub.3                                                                    6.5     9.5      4.75  4.75  4.75 11.5  6.5                             Y.sub.2 O.sub.3                                                                     5.0     --       4.75  4.75  4.75 --    5.0                             La.sub.2 O.sub.3                                                                    --      --      --    --    --    --    --                              Nd.sub.2 O.sub.3                                                                    (2.48)* (2.28)* (2.39)*                                                                             (2.44)*                                                                             (2.48)*                                                                             1.16  2.40                            Nb.sub.2 O.sub.5                                                                    (0.53)* (0.48)* (0.51)*                                                                             (0.52)*                                                                             (0.53)*                                                                             0.49  --                              Sb.sub.2 O.sub.3                                                                    (0.24)* (0.22)* (0.23)*                                                                             (0.24)*                                                                             (0.24)*                                                                             0.22  --                              ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Mole %                                                                        #15       #16     #17     #18   #19   #20   #21                               P827      P836    P837    P847  P805  P811  P842                              ______________________________________                                        P.sub.2 O.sub.5                                                                     60      60      60    60    60    60    59.12                           Li.sub.2 O                                                                          15      17      16.5  16.5  --    --    9.64                            Na.sub.2 O                                                                          --              --    --    --    --    4.9                             K.sub.2 O                                                                           2       0.5     0.5   0.5   15    17                                    BaO   11.5    11.5    13.5  13.5  13.5  11.5  13.73                           Al.sub.2 O.sub.3                                                                    6.5     6.0     6.0   6.0   11.5  6.5   9.07                            Y.sub.2 O.sub.3                                                                     5.0     5.0     3.5   3.5   --    5.0                                   La.sub.2 O.sub.3                                                                    --      --      --    --    --    --                                    Nd.sub.2 O.sub.3                                                                    2.42    2.39    2.38  3.02  1.23  1.28                                  Nb.sub.2 O.sub.5                                                                    --      --      --    --    0.52  0.54  0.6                             Sb.sub.2 O.sub.3                                                                    --      --      --    --    0.23  0.25  0.56                            SiO.sub.2                                     1.23                            Sm.sub.2 O.sub.3                              1.96                            ______________________________________                                    

For each sample 1-21, an identifying P number was assigned. The index ofrefraction (Nd) was measured for each glass sample, and the values areset forth in Table IV.

                  TABLE IV                                                        ______________________________________                                                   #1     #2     #3   #4   #5   #6   #7                                          P803   P781   P786 P787 P788 P789 P790                             ______________________________________                                        Nd (measured)                                                                            1.559  1.554  1.548                                                                              1.548                                                                              1.560                                                                              1.555                                                                              1.560                            ______________________________________                                                   #8     #9     #10  #11  #12  #13  #14                                         P791   P793   P794 P795 P796 P809 P817                             ______________________________________                                        Nd (measured)                                                                            1.559  1.546  1.550                                                                              1.549                                                                              1.548                                                                              1.548                                                                              1.559                            ______________________________________                                                   #15    #16    #17  #18  #19  #20  #21                                         P827   P836   P837 P847 P805 P811 P842                             ______________________________________                                        Nd (measured)                                                                            1.558  --     --   --   1.535                                                                              1.539                                                                              --                               ______________________________________                                    

In addition, properties of the glass samples were calculated based onthe compositional characteristics of each sample. These calculatedproperties include the index of refraction (Nd) for each sample, whichwas compared to the measured values. The coefficient of thermalexpansion (α), W_(o), representing thermo-optical, and the change in therefractive index over time (dn/dT) were calculated. Finally, thenon-linear index (N₂) was calculated. The calculated values determinedfor these properties for each test sample are listed in the followingTable V.

                  TABLE V                                                         ______________________________________                                        Calculated  #1     #2     #3   #4   #5   #6   #7                              Properties  P803   P781   P786 P787 P788 P789 P790                            ______________________________________                                        Nd (× 10.sup.-7 /°C.)                                                        1.547  1.546  1.546                                                                              1.545                                                                              1.562                                                                              1.552                                                                              1.560                           α     88     89      92   91   87   86  85                              w (× 10.sup.-7 /°C.)                                                         46     48      44   46   49   49  49                              dn/dT (× 10.sup.-7 /°C.)                                                     -2      0      -6   -3    0    1   1                              N.sub.2 (10.sup.-13 eus)                                                                  1.25   1.24   1.24 1.24 1.40 1.35 1.44                            ______________________________________                                        Calculated  #8     #9     #10  #11  #12  #13  #14                             Properties  P791   P793   P794 P795 P796 P809 P817                            ______________________________________                                        Nd (× 10.sup.-7 /°C.)                                                        1.555  1.544  1.550                                                                              1.549                                                                              1.549                                                                              1.541                                                                              1.546                           α     86     94      91   90   89   100 89                              w (× 10.sup.-7 /°C.)                                                         47     44      44   44   44   37  44                              dn/dT (× 10.sup.-7 /°C.)                                                      0     -7      -6   -5   -5   18  -5                              N.sub.2 (10.sup.-13 eus)                                                                  1.37   1.24   1.34 1.34 1.33 1.23 1.25                            ______________________________________                                        Calculated  #15    #16    #17  #18  #19  #20  #21                             Properties  P827   P836   P837 P847 P805 P811 P842                            ______________________________________                                        Nd (× 10.sup.-7 /°C.)                                                        1.546  1.546  1.546                                                                              1.548                                                                              1.536                                                                              1.538                                α     91     90      94   93   117  118                                 w (× 10.sup.-7 /°C.)                                                         42     44      39   40   13   10                                  dn/dT (× 10.sup.-7 /°C.)                                                     -8     -5     -12  -10  -50  -53                                  N.sub.2 (10.sup.-13 eus)                                                                  1.24   1.25   1.23 1.25 1.18 1.27                                 ______________________________________                                    

EXAMPLE 1

A sample of glass P8O3 was treated in a molten salt bath consisting of30% by weight NaNO₃ at 610 degrees F. for 18 hours. After thistreatment, the glass was examined and was found to have developed asignificant stress layer at the surface. This layer was estimated to beapproximately 100 micrometers thick.

EXAMPLE 2

A sample of glass P8O3 was treated in a molten salt bath consisting of22% by weight NaNO₃ plus 78% by weight KNO₃ at 610 degrees F. for 18hours. After this treatment, the glass was found to have a significantstress layer at the surface. This layer was estimated to beapproximately 150 micrometers thick.

EXAMPLE 3

A series of P8O3 glass samples were heated for 0, 19, 42 and 64 hoursrespectively at 610 degrees F. in a 22% by weight NaNO₃ plus 78% byweight KNO₃ bath. A significant stress layer was observed at the surfaceof all of the samples treated in the salt bath. These samples were thenheated to various temperatures and quenched in cold water. Thetemperatures at which the samples fractured were recorded. It was foundthat the thermal shock resistance of the samples increased significantlyas the ion-exchanged time increased as follows:

    ______________________________________                                        Heat Treatment                                                                          Thickness of  Temperatures at Which                                 Time in   Observed      Samples Fractured Upon                                Salt Bath Stress Layer  Quenching Into Water                                  ______________________________________                                         0 hrs    --             78° C.                                        19 hrs    180 micrometers                                                                             153° C.                                        42 hrs    280 micrometers                                                                             227° C.                                        64 hrs    350 micrometers                                                                             259° C.                                        ______________________________________                                    

EXAMPLE 4

Laser rods 0.25 inches in diameter by 3.25 inches in length wereprepared from the glass P8O3. These rods were finished to opticalquality in the customary manner. The laser rods were then ion-exchangedin a molten salt bath consisting of 22% by weight NaO₃ plus 78% byweight KNO₃ at 610 degrees F. for 144 hours. The rods were then testedin the laser cavity with the feedback mirrors separated from the laserrods. The thermal distortion of the laser rods was estimated bymeasuring the divergence of the laser beam as a function of time whenthe laser was rapidly pulsed. The beam divergence of the laser beamusing this glass was found to be quite good. This is because of the verylow value (near zero) of dn/dT for this glass. Some increase in the beamdivergence was observed during pumping since the value of dn/dT was notsufficiently negative to compensate completely for the high positivecoefficient of thermal expansion of the glass.

EXAMPLE 5

The overall efficiency for converting stored electrical energy intolaser light at 1060 nanometers for the laser rods tested in thepreceding example (Example 4) was measured as a function of the pumpenergy. The results of this test of these samples, as well as forseveral other glasses, are plotted in FIG. 1 of the drawings.

The overall efficiency at a pump energy of 40.5J was approximately 1.7%.Also, the slope efficiency, that is, the incremental efficiency abovethreshold, was found to be 2.4%. This was considered quite good forsmall laser rods.

EXAMPLE 6

Laser rods 0.25 inches in diameter by 3.25 inches in length wereprepared for glasses P817, P827, P836 and P847. The rods wereion-exchanged in a molten salt bath containing approximately 22% byweight NaNO₃ plus 78% by weight KNO₃ at 610 degrees F. for 144 hours.The rods were tested for lasing efficiency and for thermal distortion asdescribed in the previous examples. The results of these tests aresummarized in the following table:

    ______________________________________                                                            Lasing    Relative                                        Example No.                                                                            Glass No.  Slope Eff.                                                                              Thermal Distortion                              ______________________________________                                        1        803        2.4%      Moderate                                        14       817        2.6%      Moderate                                        15       827        2.2%      Moderate                                        16       836        1.8%      Moderate                                        18       847        --        Moderate                                        ______________________________________                                    

All the samples gave a relatively good slope energy efficiency andshowed only a moderate amount of beam divergence. In the case of sampleP803, this relatively low beam divergence is due to the low value ofdn/dT expected for these glasses. The low values of dn/dT are in partdue to the high amounts of BaO present in the glasses. Even lowerthermal distortion glasses may be obtained by further substitutingheavier alkali oxides for Li₂ O in the glass. However, as will beunderstood from the explanation of the present invention, this can onlybe done to a limited extent because of the undesirable effects on theability to ion-exchange the glasses.

EXAMPLE 7

Samples of glasses P781, P786, P787, P788, P790, P791, P793, P794, P795and P796 were treated in a molten salt bath consisting of 78% by weightKNO₃ plus 22% by weight NaNO₃ at 610 degrees F. for times ranging from24 to 48 hours, as shown in the following table. After treatment, thesamples were examined and were found to have developed significantstress layers at the surface. The thicknesses of the stress layers wereestimated and were given in the following table.

    ______________________________________                                                       Thickness of Stress Layer                                                     Micrometers After Holding                                      Example No.                                                                              Glass No. 24 hrs     46 hrs                                                                              48 hrs                                  ______________________________________                                        2          P781      150        210   200                                     3          P786       95        160   150                                     4          P787      --         190   190                                     5          P788      --         --    150                                     6          P789      --         --    210                                     7          P790      --         --    210                                     8          P791      --         --    210                                     9          P793      --         --    220                                     10         P794      --         --    190                                     11         P795      --         --    190                                     12         P796      --         --    190                                     ______________________________________                                    

The intensity of the stress in all samples was quite high. Also, thestress layers were quite thick in all samples. Therefore, significantstrengthening would be expected.

EXAMPLE 8

Samples of glasses P817, P827, and P836 were treated in a molten saltbath containing approximately 78% by weight KNO₃ plus 22% by weightNaNO₃ at 600 degrees F. After treatment, all three samples showedsignificant stress layers as shown in the following table:

    ______________________________________                                        Example                                                                              Glass    Thickness of Stress                                           No.    No.      Layer After Treatment                                                                         % Li.sub.2 O                                                                         % K.sub.2 O                            ______________________________________                                        14     P817     120 micrometers 16     1                                      15     P827      90 micrometers 15     2                                      16     P836     150 micrometers 17     0.5                                    ______________________________________                                    

The results show the detrimental effect on ion exchange whensubstituting K₂ O for Li₂ O in the glass.

EXAMPLE 9

A filter glass element was formed from glass P842 which was treated in amolten salt bath consisting of 30% by weight NaNO₃ at 605 degrees F. for48 hours. After treatment, the glass showed a significant stress layerapproximately 85 micrometers thick. The glass showed no detectablecorrosion of the surface by the salt bath.

EXAMPLE 10

A sample of glass P842 was used to form a filter glass element which wastreated in a molten salt bath containing 22% by weight NaNO₃ plus 78% byweight KNO₃ at 610 degrees F. for 18 hours. A significant stress layerwas produced.

EXAMPLE 11

A filter glass element was formed from glass P842 which was treated in amolten salt bath containing 15% by weight NaNO₃ plus 85% by weight KNO₃plus 5% Ca(NO₃)₂ at 590 degrees F. for six days. A significant stresslayer approximately 150 micrometers was produced.

EXAMPLE 12

The ion exchange in terms of stress for each sample was ranked as setforth in the following table:

    ______________________________________                                        1     2       3       4     5       6      7                                  good  good    good    good  good    good   good                               8     9       10      11    12      13     14                                 good  good    good    good  good    good   good                               15    16      17      18    19      20     21                                 good  good    good    --    very low                                                                              very low                                                                             good                               ______________________________________                                    

What is claimed is:
 1. A phosphate glass article for use with a laser,said phosphate glass article comprising a compression surface layer andan inner tension region, the composition of said phosphate glass articleconsisting essentially of at least about 50 mole percent P₂ O₅, about 5to about 30 mole percent Li₂ O, at least 0.03 mole percent of a laserglass dopant, less than about 25 mole percent MO, where M is selectedfrom the group consisting of Be, Mg, Ca, Sr, Ba and Zn and combinationsthereof, at least about 5 mole percent X₂ O₃, where X is selected fromthe group consisting of Al, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, and Lu and combinations thereof, said compressionsurface layer having a lower lithium ion concentration than said innertension region, said compression surface layer further having a greaterconcentration of an ion than the concentration of said ion in said innertension region, and said ion having an ionic radius greater than theionic radius of lithium.
 2. A phosphate glass article as defined inclaim 1, wherein the phosphate glass article further contains SiO₂.
 3. Aphosphate glass article as defined in claim 2, wherein the phosphateglass article contains less than about 1.5 mole percent SiO₂.
 4. Aphosphate glass article for use with a laser, said phosphate glassarticle comprising a compression surface layer and an inner tensionregion, the composition of said phosphate glass article consistingessentially of at least about 50 mole percent P₂ O₅, a combined molepercent of 5 percent or greater of Al₂ O₃, La₂ O₃, and Nd₂ O₃, fromabout 0.0 to about 10 mole percent of Na₂ O or K₂ O, from about 5 toabout 30 mole percent Li₂ O, less than about 25 mole percent MO, where Mis selected from the group consisting of Be, Mg, Ca, Sr, Ba and Zn andcombinations thereof, and an effective amount of a solarizationinhibitor to inhibit solarization of said phosphate glass article, saidcompression surface layer having a lower lithium ion concentration thansaid inner tension region, said compression surface layer further havinga greater concentration of an ion in said inner tension region, and saidion having an ionic radius greater than the ionic radius of lithium. 5.A phosphate glass article as defined in claim 4, wherein the phosphateglass article further contains SiO₂.
 6. A phosphate glass article asdefined in claim 5, wherein the phosphate glass article contains lessthan about 1.5 mole percent SiO₂.